1. Field of the Invention
The present invention relates generally to a communication system, and more particularly, to a method and apparatus for transmitting a control signal.
2. Description of the Related Art
The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release-10 (Rel-10) has adopted a bandwidth extension technique to support a data rate that is higher than that of the LTE Rel-8. The bandwidth extension technique is also referred to as Carrier Aggregation (CA). With CA, it is possible to increase the data rate in proportion to the extended bandwidth, as compared to the single carrier data transmission of an LTE Rel-8 terminal (also referred to herein as User Equipment (UE)).
In CA, each band is referred to as a Component Carrier (CC). The LTE rel-8 UE is configured to operate with one uplink and one downlink CC. One downlink CC and one uplink CC, which are linked with each other in association with a System Information Block 2 (SIB-2), is referred to as cell. The information about the SIB-2 association relationship between the uplink and downlink CCs is transmitted in a UE-specific signal. The UE that supports CA is capable of receiving downlink data and transmitting uplink data through multiple serving cells. In Rel-10, when it is difficult to transmit Physical Downlink Control Channel (PDCCH) to a specific UE through a specific serving cell, the base station (also referred to herein as an evolved Node B (eNB)) is capable of transmitting PDCCH through another serving cell with the configuration of Carrier Indication Field (CIF). The CIF informs that the PDCCH indicates Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH) of another serving cell. The CIF can be configured to the UE capable of CA. The CIF is designed such that a specific serving cell is capable of indicating another serving cell, with 3 extra bits added to the PDCCH information. The CIF is transmitted only when cross carrier scheduling is applied, and thus, if no CIF is transmitted, the cross carrier scheduling is not applied. If it is included in a downlink (DL) assignment, the CIF indicates the serving cell through which the PDSCH scheduled in the DL assignment is to be transmitted. If it is included in an uplink (UL) grant, the CIF indicates the serving cell through which the PUSCH scheduled in the UL grant is to be transmitted. When the cross carrier scheduling is supported, a specific serving cell has to transmit PDCCH for scheduling data in other serving cells. Accordingly, the specific serving cell needs large amounts of resources for PDCCH transmission.
The LTE Rel-10 standard specifies 13 Downlink Control Information (DCI) formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3, 3A, and 4. Among them, DCI formats 0 and 4 are the formats for a UL grant addressed to a UE. The DCI formats 3 and 3A are used for Transmit Power Control (TPC) command transmission for Physical Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH). The rest of the DCI formats are used for DL allocation. Particularly, the DCI format 1A is the format for supporting compact downlink scheduling and used for fall back in the case of missing other downlink DCI format. The DCI format 0 and 1A are configured as the same size, and 1 bit of the DCI formats 0 and 1A are used as a flag to discriminate between the DCI formats 0 and 1A.
According to Rel-10, when performing blind decoding, the UE assumes the DCI has one of three sizes. The UE first attempts blind decoding based on the assumption that the size of the received DCI matches the size of DCI formats 0 and 1A (referred to herein as a first size). Next, the UE attempts blind decoding based on the assumption that the size of the received DCI matches the size of another DCI format, with the exception of formats 0 and 1A, according to the transmission mode (referred to herein as a second size). When the specific serving cell supports uplink Single User Multiple Input Multiple Output (UL SU-MIMO), the UE further attempts blind decoding based on the assumption that the size of the received DCI matches the size of the DCI format 4 (referred to herein as a third size).
Table 1 shows the sizes of the DCIs for blind decoding of the UE capable of CA.
TABLE 1FirstSecondThirdsizesizesizeSearch SpaceDL Tx modeformatformatformatCommon Search Space1 to 71A/01C—UE-specific Search Space1, 2, 71A/01431A/02A441A/02451A/01D461A/01B4
The blind decoding in a CA situation is described with reference to Table 1. All UEs decoding PDCCH in the Primary Cell (PCell) attempt blind decoding to 6 candidates in the common search space with the assumption of DCI format sizes matching the first size and second size, regardless of the DL transmission mode. That is, the blind decoding is attempted 12 times. In Table 1, the UE decoding PDCCH in the PCell and Secondary Cell (SCell) assumes that the DCI format having a size that matches the second size depends on the DL transmission mode.
With respect to DL transmission mode 1, 2, or 7, the blind decoding is attempted with the assumption that the DCI format having a size that matches the second size is DCI format 1. In the case of DL transmission mode 3, the blind decoding is attempted with the assumption that the DCI formation having a size that matches the second length is DCI formation 2A. In the case of DL transmission mode 4, the blind decoding is attempted with the assumption that the DCI formation having a size that matches the second length is DCI formation 2. In the case of DL transmission mode 5, the blind decoding is attempted with the assumption that the DCI formation having a size that matches the second length is DCI formation 1D. In the case of DL transmission mode 6, the blind decoding is attempted with the assumption that the DCI formation having a size that matches the second length is DCI formation 1B.
Since the total number of candidates for blind decoding in the UE-specific search space is 16 and, since when the DCI format size matches the first size according to the DL transmission mode as described above, the blind decoding is attempted with the assumption of the second size, the blind decoding is attempted up to 32 times. When the blind decoding to the DCI format 4 of the serving cell supports UL SU-MIMO, the UE has to attempt 16 additional blind decodings. Assuming the number of SCells is X and the number of serving cells supporting UL SU-MIMO is Y, the UE capable of CA has to attempt blind decoding (44+32*X+16*Y) times in the blind decoding procedure of the common search space and UE-specific search space.
In LTE Rel-10, the PDCCH transmission resource is allocated in units of Control Channel Elements (CCEs), and 1 CCE consists of 36 Resource Elements (REs). For PDCCH transmission, 4 or 8 CCEs can be allocated in the common search space and 1, 2, 4 or 8 CCEs can be used in the UE-specific search space. As the allocated transmission resource increases, the code rate of PDCCH decreases so as to transmit data to the UE with higher reliability. However, the Rel-10 system, which adopts the cross carrier scheduling and CA, has a drawback in that a large PDCCH transmission resource requirement in a specific cell, or a large number of UEs requiring PDCCH transmission within the cell, causes a shortage of PDCCH transmission resources.